If you pass the array as a reference, yes, the array elements can be modified. But if you pass the array directly, a copy will be made, and only the copies will be modified, the original array will not be changed:

my @a = [ 1 .. 10 ];
by_value ( 5, @a );  # @a still is [1..10]
by_ref( 5, \@a );    # @a will be [120, 240,... ]

sub by_value
{
   my ( $time, @a ) = @_;
   if ( $time == 0 ) return;
   foreach my $b ( @a ) { $b *= $time; }
   by_value( $time--, @a );
}
   

sub by_ref
{
   my ( $time, $a_ref ) = @_;
   if ( $time == 0 ) return;
   foreach my $b ( @$a_ref ) { $b *= $time; }
   by_ref( $time--, $a_ref );
}
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